In a number of processes, such as the refining of crude oil, the purification of natural gas and the production of synthesis gas from, for example, fossil fuels, sulfur containing gas, in particular H.sub.2 S containing gas, is released. On account of its high toxicity and its smell, the emission of H.sub.2 S is not permissible.
The best known and most suitable process for recovering sulfur from hydrogen sulfide is the so-called Claus process. In this process hydrogen sulfide is converted by oxidation to a considerable extent into elemental sulfur; the sulfur thus obtained is separated by condensation. The residual gas stream (the so-called Claus residual gas) still contains some H.sub.2 S and SO.sub.2.
The method of recovering sulfur from sulfur containing gases by the so-called Claus process is based on the following reactions: EQU 2H.sub.2 S+3O.sub.2.fwdarw. 2H.sub.2 O+2SO.sub.2 (1) EQU 4H.sub.2 S+2SO.sub..fwdarw..sup..rarw. 4H.sub.2 O+6/nS.sub.n (2)
Reactions (1) and (2) result in the main reaction: EQU 2H.sub.2 S+O.sub.2.fwdarw. 2H.sub.2 O+2/nS.sub.n (3)
A conventional Claus unit--suitable for processing gases having an H.sub.2 S content of between 50 and 100%--comprises a burner with a combustion chamber, the so-called thermal stage, followed by a plurality of reactors--generally two or three--filled with a catalyst. These last stages constitute the so-called catalytic stages.
In the combustion chamber, the incoming gas stream, which is rich in H.sub.2 S, is combusted with an amount of air at a temperature of ca. 1,200.degree. C. This stoichiometric amount of air is adjusted so that one third of the H.sub.2 S is fully combusted to form SO.sub.2 in accordance with the following reaction EQU 2H.sub.2 S+3O.sub.2.fwdarw. 2H.sub.2 O+2SO.sub.2 (1)
After this partial oxidation of H.sub.2 S the non-oxidized part of the H.sub.2 S (i.e. basically two-thirds of the amount offered) and the SO.sub.2 formed react further as to a considerable portion, in accordance with the Claus reaction: EQU 4H.sub.2 S+2SO.sub.2.fwdarw..sup..rarw. 4H.sub.2 O+3S.sub.2 (2)
Thus, in the thermal stage, approximately 60% of the H.sub.2 S is converted into elemental sulfur.
The gases coming from the combustion chamber are cooled to about 160.degree. C. in a sulfur condenser, in which the sulfur formed is condensed, which subsequently flows into a sulfur pit through a siphon.
The non-condensed gases, in which the molar ratio of H.sub.2 S:SO.sub.2 is unchanged and so still 2:1, are subsequently heated to about 250.degree. C. and passed through a first catalytic reactor in which the equilibrium EQU 4H.sub.2 S+2SO.sub.2.fwdarw..sup..rarw. 4H.sub.2 O+6/nS.sub.n (2)
is again established.
The gases coming from this catalytic reactor are subsequently cooled again in a sulfur condenser, in which the liquid sulfur formed is recovered and the remaining gases, after being re-heated, are passed to a second catalytic reactor.
When the gaseous feedstock contains H.sub.2 S concentrations of between about 15 and 50%, the above described `straight-through` process is not used, but instead a variant thereof, the so-called `split-flow` process is employed. In the latter process one-third of the total amount of feedstock is passed to the thermal stage and combusted completely to SO.sub.2 therein. Two-thirds of the feedstock is passed direct to the first catalytic reactor, by-passing the thermal stage.
When the feedstock contains H.sub.2 S concentrations of between 0 and 15% the Claus process can no longer be used. The process then used is, for example, the so-called Recycle Selectox process, in which the feedstock is passed with an adjusted amount of air into an oxidation reactor, the so-called oxidation stage. The reactor contains a catalyst which promotes the oxidation of H.sub.2 S to SO.sub.2, and the amount of oxidation air is adjusted so that an H.sub.2 S:SO.sub.2 ratio of 2:1 is established, whereafter the Claus reaction proceeds. The gas from the oxidation reactor is cooled in a sulfur condenser, in which the sulfur formed is condensed and discharged.
To dissipate the reaction heat generated in the oxidation reactor, a portion of the gas stream coming from the sulfur condenser is re-supplied to the oxidation reactor.
It is clear that in the Recylce Selectox process, the oxidation stage, which is catalytic and does not lead to high temperatures, is equivalent to the thermal stage in the Claus process.
In the following, both stages are referred to as oxidation stages.
Depending on the number of catalytic stages, the sulfur recovery percentage in a conventional Claus unit is 92-97%.
By known processes, the H.sub.2 S present in the residual gas from the Claus reaction is converted, by combustion or some other form of oxidation, into SO.sub.2, whereafter this SO.sub.2 is emitted to the atmosphere. This has been permissible for low concentrations or small amounts of emitted SO.sub.2 for a long time. Although SO.sub.2 is much less harmful and dangerous than H.sub.2 S, however, this substance is also so harmful that its emission is also limited by ever stricter environmental legislation.
As has been observed, in the Claus process as described above, in view of the equilibrium reaction which occurs, the H.sub.2 S:SO.sub.2 ratio plays an important role. In order to obtain an optimum conversion to sulfur, this ratio should be 2:1. Generally speaking, this ratio is controlled by means of a so-called H.sub.2 S/SO.sub.2 residual gas analyzer. This analyzer measures the H.sub.2 S and SO.sub.2 concentrations in the residual gas. A controller then maintains the ratio of 2:1 constant on the basis of the equation EQU [H.sub.2 S]-2[SO.sub.2 ]=0,
by varying the amount of combustion air, depending on the fluctuations in the gas composition and the resulting deviation in the above equation. Such a control of the process, however, is highly sensitive to these fluctuations.
Furthermore, the sulfur recovery efficiency (calculated on the amount of H.sub.2 S supplied) is no higher than 97%, and so the gas flowing from the last catalytic stage--the residual gas--still contains substantial amounts of H.sub.2 S and SO.sub.2, determined by the Claus equilibrium, and this in a molar ratio of 2:1.
The amount of H.sub.2 S present in the residual gas can be separated by absorption in a liquid.
The presence of SO.sub.2 in the residual gas, however, is a disturbing factor during the further processing thereof and must therefore be removed prior to such further processing. This removal and hence the after-treatment of the gas is complicated.
The great disadvantage of the presence of SO.sub.2 is that this gas reacts with conventional liquid absorbents to form undesirable products. To prevent undesirable reactions of the SO.sub.2, therefore, the SO.sub.2 is catalytically reduced with hydrogen to form H.sub.2 S over an Al.sub.2 O.sub.3 supported cobalt molybdenum catalyst in accordance with the so-called SCOT process. The total amount of H.sub.2 S is subsequently separated by liquid absorption in the usual manner.
In accordance with another method, for example, the BSR Selectox process, after reduction of the SO.sub.2 in residual gas to H.sub.2 S and after condensation of the water vapour, the gas is passed into an oxidation reactor, as in the Recycle Selectox process. The oxidation air is adjusted so that an H.sub.2 S:SO.sub.2 ratio of 2:1 is adjusted, whereafter the Claus reaction proceeds. Both in the SCOT process and in the BSR Selectox process, the removal of SO.sub.2 from the residual gas is a relatively expensive operation.
The above-described after-treatment of the gases, carried out by means of a so-called Tail Gas Treater, which involves an investment of another 50-100% of the cost of the preceding Claus converter, can result in an increase of the sulfur recovery efficiency of up to 98-99.8%.
In NL-A-6901632, it is proposed that the ratio of hydrogen sulfide to sulfur dioxide in the above reaction (1) be adjusted to between 2.5:1 to 4.0:1.
In NL-A-7603622, it is proposed that the above reaction (1) be conducted with an insufficient amount of oxygen, that is to say, with a proportion of oxygen less than required to combust one third of the quantity of H.sub.2 S supplied to the burner. Thus, relative to H.sub.2 S, a substoichiometric amount of SO.sub.2 is formed in reaction (1), so that ultimately, in view of the equilibrium reaction (2), the resulting ratio of H.sub.2 S:SO.sub.2 becomes higher than 2:1.